Spark gap device



Patented Aug. 21, 1951 SPARK GAP DEVICE TomL. Dyer, Jr., Pittsburgh, Pa assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 25, 1950, Serial No. 146,275

9 Claims.

The present invention relates to spark gap devices for valve type lightning arresters and, more particularly, to an improved spark gap which has a low and consistent sparkover or breakdown voltage and impulse ratio, and which is especially suitable for use in lightning arresters intended for the protection of rotating machines, such as motors and generators.

Valve type lightning arresters consist essentially of a spark gap, or a series of spark gaps, connected in series with a resistance element having valve characteristics, that is, a resistor which has very high resistance under normal voltage conditions, but which sharply reduces its resistance under a predetermined excess voltage, to permit a surge to be discharged to ground with low discharge voltage, and which is capable of thereafter increasing its resistance to reduce the power follow current to 'a small value. The series gap device normally insulates the arrester from the line to which it is'connected, but sparks over under a predetermined excess voltage to connect the arrester betweenline and ground to allow a voltage surge to be discharged to ground. After discharge of the surge, the valve element of the lightningv arrester reduces the power current, which tends to follow the surge, to a small value which is interrupted by the gap to again insulate the arrester from the line.

It will be apparent that in order .to obtain the greatest degree of protection, the gap should have a low surge sparkover voltage, so as to break down as rapidly as possible when a voltage surge occurs, and that the surge sparkover should be consistentonsuccessive operations, so that the :behavior of the gap can be predicted with reasonable accuracy. When a spark gap is subjected to a surge voltage, the actual voltage at which the gap sparks over is usually higher than the sparkover voltage under a normal or low-frequency voltage, because of a time lag effeet. The ratio of the surge sparkover voltage of a gap to the crest value of the 60 cycle, or normal-frequency, sparkover voltage is known as the impulse ratio of the gap, and it will be apparent that this ratio should be as close to unity as possible, and should be consistent on successive operations of the gap. The impulse ratio of a gap will, of course, vary with the wave shape of the surge voltage, and standard voltage waves are used in determining impulse ratios.

The foregoing requirements for lightning arrester spark gaps are particularly important in the case of arresters which are intended for'the protection of rotating machines. The sparkover and discharge characteristics of such arresters must be lower and more consistent than those of arresters intended for the protection of static apparatus, such as transformers and circuit breakers, because of the lower insulation level of rotating machines as compared to that of static apparatus of the same voltage class. The turnto-turn insulation of the windings of rotating machines is also relatively weak and must be protected from rapidly rising voltages. For this reason, lightning arresters for rotating machine protection are usually used in connection with capacitors, and sometimes reactors, to reduce the rate of rise of voltage surges reaching the machine, so that the windings will not be subjected to surges of steep wave front. The sparkover characteristics of arresters intended for this service, therefore, are usually determined on the basis of voltage surges rising to sparkover in ten microseconds, as compared to surges rising to sparkover in one microsecond, or less, used for arresters for other applications. When voltage surges of such relatively slow rate of rise are applied to spark gaps of conventional construction, their surge sparkover voltages and impulse ratios are undesirably high, and frequently vary widely between successive operations so that both the impulse ratios and the consistency of the gaps are unsatisfactory. Such expedients as radium treatment of the electrodes have been utilized to effect ionization of the gap and thus improve the characteristics, but it has been found that no material improvement can be obtained in this way.

The principal object of the present invention is to provide a spark gap device for valve type lightning arresters which has low and consistent impulse ratio and sparkover voltage under slowly rising voltage surges.

Another object of the invention is to provide a spark gap device which is especially suitable for use in lightning arresters intended for the protection of rotating machines, and which has low and consistent impulse ratio and sparkover voltage.

A further object of the invention is to provide a spark gap device utilizing an insulating member or button of material of high dielectric constant associated with one of the electrodes and extending into contact with the other electrode to provide an ionizin radiation under voltage surges, and in which means are provided for holding the button in good contact with the electrode and for preventing the occurrence of radio interference at normal voltage.

Other objects and advantages of the invention will be apparent from the following detailed description, taken in connection with the accompanying drawing, in which Figure 1 is a sectional plan view of a gap assembly embodying the invention taken approxi mately on the line I-I of Fig. 2;

Fig. 2 is a longitudinal sectional view of i the gap assembly of Fig. 1, and

Fig. 3 is a longitudinal sectional View showing an alternative embodiment of the invention.

Figs. 1 and 2 of the drawing show a preferred embodiment of the invention in'a'double gap assembly for use in a lightning-arrester. The double gap assembly shown has an "intermediate electrode structure consisting of two electrode members I and 2, which may be made of brass or other suitable metal. The electrodes l and 2 have fiat peripheral portions and are shaped with an annular. ridge ordishedportion, as shown in the drawing, to provide an annular sparking area, and with alargecentral opening 3. The electrode members land-2 maybe secured together at the edges, if desiredgby spot welding, or in any other suitable mannento form a unitary intermediate electrodestructure for ease of handling and -assembly. An annular spacer 4 is placed on each side ofthe intermediate electrode structure. The spacers 4 may be made of insulating material, but are preferably made of .a high resistance material to obtain proper Voltage distribution across a series of gaps. Other electrode members 5 and ii are disposed on-the opposite sides of the spacers t to complete the double gap assembly. The electrodes 5 and-8 may be disc shaped members of brass, orother suitable metal, and are spaced fromthe electrode members i and .2, respectively, --by thespacers 4 to form gaps l and 8, respectively, with-the sparking areas of the electrodes l and 2, thus providing two series-related gaps.

Aspreviously indicated, a-gap. assembly of the structure so far described willthave an undesirably high and inconsistent impulse-ratio and surge sparkover voltage, especially when subjected to a relatively slowly rising voltage surge. Inorder to reduce the impulse ratio and make it more consistent, insulatingmem-bers or blittons 9 of high dielectric constant :are utilized. The insulating buttons 9 maybemade-of any suitable material of high dielectric-constant,-but they are preferably made of rutile ceramic. Rutile is a natural mineral consisting mainly of titanium dioxide, and a ceramic .inater-ialmade of rutile has a very high dielectrioconstant, of the order of 80 or higher. The :rutilecera'mic buttons 9 are disposed in the central openings -3 of the electrodes I and 2 and extend across the gap into contact with the outer electrodes 5 and 6 at one side of the gap spaces! and Rutile ceramic buttons have been utilized-heretofore in lightning arrester spark gapsto-prov-ide an ionizing radiation and thus improvethe impulse ratio. When a high voltage is applied across a spark gap having a button .of rutile ceramic, or equivalent material,extending-across the space between the electrodes,--the high dielectric constant of the button results in practically all of the voltage being applied to the very small air gap, or film of air, between theen'd ofthe button and the adjacent electrode. The high voltage gradient in this extremely'small thickness of air overstresses the air in this'local region, and causes the emission ofa-ra diation 4 which is directed into the adjacent gap space and ionizes the air in the gap space, thus preionizing the gap so that sparkover is facilitated and time-lag eliminated, and the surge sparkover voltage, and therefore the impulse ratio of the gap, are reduced and made more consistent.

It has been found, however, that on slowly rising voltage surges, such as are encountered in lightningarresters used for rotating machine protection, it is necessary for the rutile button to make very good contact with the electrode in order to obtain the necessary ionization and the desired improvement in the impulse ratio,

and relatively-small variations in the contact -matter, it cannot be obtained merely by properly dimensioning the parts 90f the gap assembly because ;of the extremely smalljmanufacturing tolerances that would;have to be. maintained to insure'that the electrode '5 or fiflwould always lie flat on the button 9 and spacer 4 with the proper contact-between the button and the elec-, trode, which is necessary because even i slight variations in the contact between the rutile button andltheadjacent'electrode will have a marked effect on the efiicacy of the button in pre-ionizing the gap. The second problem arises from theclose contact between the button and the electrode, which results in Very high voltage gradients and is likelyto cause radio interference at'normal voltage, unless extremely good contact is'cbtained.

In accordance with the present invention, the rutile buttons 9 are designed sothat the required good contact is obtained without requiring impractically close tolerances inthe manufactureof the-parts of thegap assembly and without causingradio interference. 'Thejbuttons 3 are generally cylindrical and have-shoulders [0 which engage the electrode[ or 2.. Each button also has a central axial-recess I I. .The buttons 9 are inserted into the openingss of the. electrodes! and .2, as shown, and a helicaLcompression spring 12 is placed inthe'recesses I l. of the two-buttons 9. g It will be apparentthat thejspring I2 .urges the buttons apart in oppositedirections and holds them in good contact with .the.-..electrodes5 and B, irrespective of small 1 variations: in dimensions of the buttonsor spacers, outward:- movement of the buttons 9 being-limited by engagement of the shoulders Hlewith the electrodes I and-2, respectively. Inthis .way,good contactbetween the buttons.:9 and the electrodes! andt-is obtained inspite of unavoidable-manufacturing variations in=the dimensions of l the parts of the assembly, and low and consistent impulseratios areobtained even on-s1owly rising voltage-surges.

When' the rutile buttons are held in 'close" con tact with't'he adjoining electrodes, the problem of radio interference mentionedabovemust be solved in "order to produce a satisfactory and commercially usablespark gap; This problem results'from the high voltage gradients between the'button and the adjacent electrode, which will causeradiointerference. This problem can be overcome, and radio interferenceeliminated, if the contact between thebutton and electrode can be made very goodand is made uniform overthe entire 'en-dsurfacefofi the button. This might be done by plating the end of the button with metal. This is not asatisfactory solution, however, since the useo'f ametalcoating on the buttonreduces the amount of radiation produced sufficiently so that the button loses its effectiveness on gaps of less than a certain, more or less, critical, spacing. For this reason metal plating is not an acceptable solution of the problem of radio interference.

In accordance with the present invention, this problem is solved by accurately shaping the surface of the button to obtain very good and uniform contact between the button and the adjoining electrode over the entire area of the button which engages the electrode. In the preferred embodiment of Figs. 1 and 2, the end surface of each of the buttons 9 is formed with a shallow depression, leaving an annular surface I3 in engagement with the electrode. This surfacev I3 is ground and polished to an accurately plane surface, which is made as smooth and asaccurate as possible, preferably approaching optical smoothness. With such a surface, good and uniform contact is obtained between the entire surface of the button and the adjacent electrode, and no radio interference occurs. The use of the spring means for holding the button in good contact with the adjacent electrode, together with the ground and polished surface of the button for insuring good contact over the entire surface, thus makes possible a spark gap which has low and consistent impulse ratio and surge sparkover voltage, even on slowly rising voltage surges, so that it is very suitable for use in lightning arresters for rotating machine protection, and which will not cause radio interference. If desired, the interior surface of the recesses II and the adjacent outer surface of the button 9, including the shoulder Ill, may be metal plated-to insure good contact with the electrode I or 2, and prevent any possibility of radio interference due to high voltage gradients between the button and the electrode I or 2, or between the button and the spring I2. I

In use, the multiple gap assembly of Fig. 2, or a plurality of these assemblies disposed in a series stack, are connected in series to a line conductor I4 and a lightning arrester valve element or resistance element I5, which is connected to ground It. Any suitable or usual construction and arrangement of the complete arrester may, of course, be utilized.

An alternative embodiment of the invention is shown in Fig. 3. In this construction the intermediate electrode structure comprises two electrode members 26 and 2|, which may be generally similar to the electrodes I and 2 of Fig. 2. The spacers 22 and outer electrodes 23 and 24 may also be similar to the corresponding elements of Fig. 2. In this embodiment of the invention, however, each of the rutile buttons 25 is hollow, and a leaf spring 26 is secured to the top of the button by means of a rivet 21. The annular surfaces 28 of the ends of the button are ground and polished to an accurately plane surface, as described in connection with the embodiment of Fig. 2, and the top surfaces of the buttons may be metal plated, if desired. It will be apparent that when the gap structure of Fig. 3 is assembled, the leaf springs 26 will hold the rutile buttons 25 in good contact with the electrodes 23 and 24, and the ground and polished surfaces will insure good and uniform contact throughout the area of engagement, so that the operation of the gap to obtain low impulse ratio and to avoid radio interference is the same as previously described. This embodiment of the invention, therefore, is fully equivalent in its performance and electrical characteristics to that of Fig. 2, but may be somewhat less desirable because of the greater difficulty in assembly resulting from the larger number of separate parts.

It should now be apparent that a spark gap device has been provided for lightning arresters which has a low and consistent surge sparkover voltage, and low and consistent impulse ratio, on slowly rising voltage surges, so as to be especially suitable for use in lightning arresters intended for the protection of rotating machines. The new spark gap construction differs from previous gaps which have utilizedbuttons of rutile ceramic or similar material in providing for extremely good contact between the button and its adjacent electrode so as to obtain the maximum pre-ionizing effect in spite of variations in the dimensions of the parts of the assembly, and without causing radio interference.

Certain preferred embodiments of the invention have been shown and described for the purpose of illustration, but it will be obvious that the invention is not restricted to the particular details of construction shown and described, but is capable of various modifications and other embodiments. Thus, it will be apparent that either of the constructions shown could be applied to a single gap as well as to double gap structures. It is to be understood, therefore, that the invention is not limited to the particular embodiments shown, but in its broadest aspects, it includes all equivalent embodiments and modifications which come within the scope of the appended claims.

I claim as my invention:

1. A spark gap device comprising a pair of stationary, spaced electrodes providing a gap space between them, an insulating member of material of high dielectric constant associated with one of said electrodes at one side of the gap space and extending across the space between the electrodes into contact with the other electrode adjacent the gap space, and spring means for holding the insulating member in good contact with said other electrode.

2. A spark gap device comprising a pair of stationary, spaced electrodes providing a gap space between them, an insulating member of material of high dielectric constant associated with one of said electrodes at one side of the gap space and extending across the space between the electrodes into contact with the other electrode adjacent the gap space, and spring means for holding the insulating member in good contact with said other electrode, the surface of the insulating member in contact with the last-mentioned electrode being an accurately plane surface.

3. A spark gap device comprising a pair of stationary, spaced electrodes providing a gap space between them, an insulating member of material of high dielectric constant associated with one of said electrodes at one side of the gap space and extending across the space between the electrodes into contact with the other electrode adjacent gap space, and spring means for holding the insulating member in good contact with said other electrode, the surface of the insulating member in contact with the last-mentioned electrode being ground and polished to an accurately plane surface.

4. A spark gap device comprising a pair of stationary, spaced electrodes providing a gap space between them, an insulating member of material of high dielectric constant associated with one of said electrodes at one side of the gap 7 space andextendin'g across the space between the-electrodes into contact with the other-elec-'- trode adjacent the gap space, said insulating member having a recess therein,'and a spring in said recess for holding the insulating memher in good contact with said otherelectr'oda -5. A spark gap device comprising a pair of stationary, spaced electrodes providing a-gap space between them, an insulating member of material of high dielectric constant associated with oneof said electrodes at one side of the gap space and extending across the space between the electrodes into contact with the other electrode adjacent the gap space, said insulating member having a rgcess therein, and a spring in said recess-for holding the insulating member in good contact withsaid other electrode, the surface of the insulating member in contact with thelast-meritioned electrode being ground andpolished to'an accurately plane surface.

6. A double gap structure comprising an intermediate electrode structure, spacing means on each side of the intermediate electrode structure, outer electrode members on each side of the intermediate electrode structure and spaced therefrom by the spacing means to form two series-related spark gaps, two insulating members of material of high dielectric constant associated with the intermediate electrode structure at one side of the spark gaps and extending in opposite directions into contact with the outer electrode members, the insulating members having axial recesses therein, and a compression spring disposed in said recesses for urging the insulating members apart into good contact with the outer electrode members.

7. A double gap structure comprising an intermediate electrode structure, spacing means on each side of the intermediate electrode structure, outer electrode members on each side of the intermediate electrode structure and spaced therefrom by the spacing means to form two series-related spark gaps, two insulating members of material of high dielectric constant associated with the intermediate electrode structure at one side of the spark gaps and extending electrode members, the

8 in opposite directions into contact with the outer insulating members havingaxialrecesses therein, and a compression spring disposed in said recesses for urging the insulating members apart into good contact with the outer electrode members, the surfaces of the insulating members in contact with the outer electrode -members being ground and polished to accurately plane surfaces.

8 A spark'g'ap device comprising a pair of stationary, spaeed electrodes providing a gap space between them, an insulating member of material of high dielectric constant associated with one of said electrodes at one side of the gap space and extending acrossthe space between the electrodesinto contact with the other electrode adjacent tlie gapspace, and a leaf spring interposed between the first-mentioned electrode and the insulating member for holding the insulating member in good-contact with said other electrode'.

9. A spark gap device comprising a pair of stationary, spaced electrodes providing a gap space between them, an insulating member of material of high dielectric constant associated with one of saidelectrodes at one side of the gap space and extending across the space between the electrodes into contact with the other electrodeadjacent the gap space, and a leaf spring interposed between the first-mentioned electrode and the insulating member for holding the insulating member-in good contact with said other electrode, the surface of the insulating member in contact with the last-mentioned electrode being ground and polished to an accurately plane surface.

TOM L. DYER, JR.

REFERENCES CITED The following references are of record in the file of this patent:

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